1. Field of the Invention
This invention relates to a fluid-cooled mold for use in the production of a quartz crucible, and more particularly to a fluid-cooled mold having an optimized mold structure.
2. Description of the Related Art
A quartz glass crucible is used for pulling a silicon single crystal as a semiconductor material, a silicon crystal as a solar battery material or the like. For example, the silicon single crystal is mainly produced by a method in which a polycrystalline silicon lump charged in the quartz glass crucible is melted by heating to form a silicon melt and then a seed crystal is immersed in the silicon melt and pulled therefrom. The silicon crystal as a solar battery material is low in single-crystallinity as compared with single silicon crystal as a semiconductor material, but is produced by the same pulling method.
As a method of producing a quartz glass crucible is known a rotating mold method. In this method, a rotatable bowl-shaped mold is used, and raw quartz powder is deposited on an inner surface of the mold at a given thickness along the inner surface by utilizing centrifugal force generated during the rotation of the mold. Subsequently, the quartz powder is melted and vitrified by heating to a temperature (about 2000° C.) higher than a melting point (melting temperature) through arc discharge of an electrode(s) disposed above the mold and around the rotation central axis of the mold to form a glass crucible having a form along the inner surface of the mold, and thereafter the resulting glass crucible is cooled and taken out from the mold.
A fluid-cooled mold has hitherto been known as a mold used for producing the quartz glass crucible by the rotating mold method. For example, JP-A-H11-43394 discloses that raw quartz powder is charged into a rotating stainless-steel fluid-cooled mold and melted through arc discharge to produce a quartz glass crucible. Similarly, JP-A-2002-154890 discloses that quartz powder is charged into a rotating stainless-steel fluid-cooled mold and shaped through arc discharge under a reduced pressure to produce a quartz glass crucible.
In the conventional stainless-steel fluid-cooled mold, the heat damage of the mold is prevented by forming a space for flowing of a cooling fluid inside a bottom portion and peripheral wall portion of the mold and cooling the bottom portion and peripheral wall portion of the mold with the cooling fluid under an environment by heating at a higher temperature. The inner surface of the mold is usually cooled down to about 100° C. On the other hand, the raw quartz powder deposited on the inner surface of the mold is melted and vitrified by heating to a temperature above the melting point through arc discharge.
In the cooling structure of the conventional fluid-cooled mold, the quartz powder deposited on the inner surface of the mold is melted and vitrified by heating above the melting point at an outer surface side opposite to the inner surface of the mold, while a large portion of the quartz powder located at the inner surface side of the mold remains at an unmelted state without vitrification because heat in such a portion is removed by the cooling of the mold. In order to form a vitrified layer having a target thickness with the conventional fluid-cooling mold, the quartz powder is used in a quantity larger by about 2 times than the weight required for target thickness. Therefore, there is a problem that the quantity of quartz powder for the formation of a product is small as compared with the quantity used and the loss in the quantity of the quartz powder is large.
On the other hand, there is known a carbon mold, the whole portion of which is made of carbon. Although carbon is high in heat resistance as compared with stainless steel, it is subjected to heat damage due to oxidative consumption if used at a high temperature for an extended period of time. There is a problem that the heat-damaged mold causes failures in the shape of the quartz glass crucible and thus the whole of the expensive mold is replaced with a new mold, which significantly reduces the economic efficiency.